WASHINGTON—President Trump’s trade battle with China is shaping up as a definitive issue in the 2020 campaign, with shaken financial markets and fears of an economic slowdown testing the Republican’s insistence that his blunt-force trade posture will be an asset as he seeks re-election.

Democratic presidential hopefuls including front-runner
Joe Biden
already see vulnerabilities for Mr. Trump in the effects at home, including businesses and consumers hit by tariffs and farmers who can’t sell their crops to China because of Beijing’s retaliatory measures.

“It’s crushing the American farmer,” the former vice president said Tuesday in Warren County, Iowa. “How many farmers across this state, across this nation, have to face the prospect of losing everything, losing their farm because of these tariffs?”

Mr. Trump has so far remained unfazed. At the Group of Seven summit in Biarritz, France, on Sunday, Mr. Trump appeared to show a glimmer of regret about escalating trade tensions with China. However, hours later, White House press secretary
Stephanie Grisham
doubled down on the president’s commitment, saying Mr. Trump “regrets not raising the tariffs higher.”

While there are risks to his strategy, some political analysts and pollsters contend his get-tough-on-China platform can help anchor the president’s campaign, just as it did in 2016.

“2020 is going to be framed by this issue,” said
Steve Bannon,
Mr. Trump’s former chief strategist. “It’s beyond a trade war. It’s engaging them in this economic conflict, which is trade, technology and currency. He’s got to be out there every day pounding it.”

Mr. Trump, however, could be vulnerable if pressing the trade war against China yields no obvious results. Employment growth in manufacturing has slowed over the past year. Measures of industrial production are also slumping, indicating a manufacturing economy under pressure.

What is more, the impact of tariffs on consumers is just beginning to be felt.
JPMorgan Chase
& Co. estimates the average U.S. household will have to pay $1,000 a year in added costs once all scheduled levies are implemented.

The government typically uses three tools to help ward off a recession. But with these already in play to help prop up the economy, the Trump administration may have reason to worry. WSJ's Gerald F. Seib explains. Photo: AP

Mo Elleithee, executive director of the Georgetown Institute of Politics and Public Service and a former top Democratic National Committee official, believes Mr. Trump’s aggressive posture with China could backfire as consumers start paying higher prices.

“While it may help the president with some people who like the bravado of it, the economic bottom line for a lot of people is very shaky,” Ms. Elleithee said. “You’ve got some supporters of the president saying this pain is your patriotic duty, but I’m not sure that is something a lot of people are going to buy into a few months down the road.”

Democrats overall have been divided on trade, with top-echelon candidates like Sens. Bernie Sanders and Elizabeth Warren holding similar views as Mr. Trump. One of Mr. Trump’s fiercest critics, Senate Democratic leader Chuck Schumer of New York, has repeatedly urged him to carry on the fight with China.

At rallies and in his comments to reporters, Mr. Trump has portrayed himself as taking on a job others didn’t have the stomach to pursue.

“This isn’t my trade war,” he told reporters on Wednesday, angrily dismissing a question about the risk of a recession. “This is a trade war that should have taken place a long time ago by a lot of other presidents.”

Looking into the sky, spreading his hands, he added: “I am the chosen one. Somebody had to do it. So I’m taking on China.” Mr. Trump later said he was joking about being the chosen one.

Within the administration, advisers who early on warned against a conflict have mostly left, giving more sway to hawks who have urged him to press forward.

And while Americans in general support free trade, people believe China has unfair trade practices with the U.S. by a 62% to 30% margin, according to a Gallup poll last year.

“China is a unique case,” said Mohamed Younis, Gallup’s editor in chief. “It’s the only country where a majority of Americans agree that they have an unfair trade relationship with the United States.”

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Mr. Trump won the presidency in 2016 in large part by narrowly flipping industrial states Michigan, Pennsylvania and Wisconsin, and GOP strategists say his message resonated with factory workers and others who believed China’s rise came at the expense of manufacturing jobs in the U.S.

But the economy’s job gains under Mr. Trump have been concentrated in professional services and health care, industries far removed from the trade war.

The brunt of the conflict has been felt by farmers, who tend to be in deeply Republican regions and so far back the president’s initiative. Their economic pain has been mitigated by billions of dollars in government aid to make up for the loss of sales to China.

The patience voters may afford Mr. Trump now could change if the overall economy begins to sour, and if voters blame the trade war for a downturn.

In recent days, as worries of a global recession have risen, the president has blasted the Federal Reserve and its chairman,
Jerome Powell,
just as White House officials have fanned out to TV news shows to argue the economy is strong and not threatened by the trade war.

“They know that’s where they are exposed and are starting a campaign against it,” said Douglas Holtz-Eakin, former chief economist for the White House Council of Economic Advisers under President
George W. Bush.
“They are pushing very hard to stop that narrative.”

Chris Krueger,
a managing director at Cowen Washington Research Group, said Democrats would rather talk about topics such as climate change and income inequality than trade. Because they generally agree with Mr. Trump’s posture toward China, he said, they are left with arguing the more nuanced point that he is going about it the wrong way.

“It’s sort of a muddled message right now,” Mr. Krueger said.

Despite their public bravado, members of Mr. Trump’s team are preparing for a slowing economy by framing the trade war as a fight for American jobs and workers. One senior Trump adviser played down the impact of market turmoil on rank-and file voters, characterizing that as an issue that largely concerns the affluent.

“Democrats want to fight for the stock market and fat cats on Wall Street? Go right ahead,” the adviser said.

Even amid slowing manufacturing, the Trump campaign thinks the president could expand the electoral map to several other states with traditional manufacturing bases like Minnesota, New Hampshire and Maine.

Mr. Bannon is among those contending the conflict will run through the campaign season.

“Trump’s not folding and this is not going to be resolved by Election Day in 2020,” he said. “This is the single most defining issue in the first half of the 21st century and it will set the tone and the direction of not just the country but the global economy. That’s why this is a fight worth having.”